1. Field of the Invention
The present invention relates to optical fibers and waveguides. More particularly, the present invention relates to corrosion-resistant optical fibers and waveguides which include thin quasi metal oxide films or reagents for improved mechanical performance.
2. Related Art
Until recently, optical fibers and waveguides were generally limited to the field of telecommunication. Correspondingly, optical fibers were used in relatively protected environments, such as subterranean cabling. Newer applications, which include fiber to the home (e.g., for cable television) and longer spans of submarine cabling, place more demanding mechanical requirements on optical fibers. Thus, it has become imperative that the fiber itself be more corrosion-resistant.
In general, an optical fiber includes a core, a cladding, and one or more protective layers. The core is the center of the optical fiber and serves as the medium for transmitting light encoded information. The cladding which surrounds the core is also transparent, but it has a lower refractive index to confine the light to the core. The core and cladding can have any of a broad range of dimensions and can be comprised of silica and other glass compounds.
The core-cladding assembly optionally has protective coatings of polymeric buffers, metals, metal oxides, or nitrides. These protective buffer materials are often carried in an outer covering layer over the cladding. Surrounding this buffer may be one or more additional protective layers which may be polymerized, braided, or woven about the fiber.
Conventional fibers can suffer from damage caused by both abrasion and corrosion (defined as zero-stress aging, or the breaking of silicon-oxygen (Si--O) bonds when a fiber is not under mechanical stress) and/or fatigue. The intrinsic strength of pristine glass optical fibers is very high, on the order of 1,000,000 psi for silica-based fibers, for example. In addition, freshly drawn fibers are usually flaw free, as has been demonstrated by their unimodal strength distribution and their inert strength close to the theoretical value. The outer surface of the cladding, however, is very brittle and prone to surface imperfections (e.g., abrasions, cracks, or flaws). Typically, optical fibers fail from these flaws when placed under sufficient tensile stress. It is well known that flaws in fibers grow subcritically prior to failure when subjected to tensile stress in the presence of environmental fluids such as water, ammonia, or other corrosive agents (e.g., acids or bases). Research indicates that the environmental fluids, for example water molecules, enter these tiny preexisting flaws or cracks of stressed fiber and react with silicon and oxygen at the opening of the crack rupturing the silicon-oxygen (Si--O) bonds. This phenomenon of subcritical crack growth is known as fatigue and greatly impacts the long-term reliability of optical fibers. Accordingly, the long-term mechanical reliability of optical fibers is governed, in part, by the rate of flaw initiation and growth. Although fiber breaks in the field are still extremely rare due to these mechanisms, the possibility appears to be greater under the more demanding conditions of new applications.
Indeed, while the latest surveys indicate that optical performance factors such as attenuation and dispersion remain important, mechanical performance is increasingly being voiced as the predominate concern. At the same time, however, applications are requiring higher fiber-count cables, shorter cable runs, and increased handling and splicing capabilities. Correspondingly, this equates for the need of smaller diametered fibers with greater mechanical reliability.
Research has focused on understanding both abrasion and corrosion resistance for some time. As a result, much effort has been devoted to the elimination of surface flaws by careful handling during and after fiber drawing, by protective coatings, and by various treatments to the cladding surface. Conventional solutions broadly fall into one of four categories: modifications of the outermost layer of the core-cladding fiber assembly (not by coating, but rather by modifying the silica itself); alterations to the polymer material of the protective layers; enhancements to the adhesion between the core-cladding assembly and the polymer protective layers; and developments of new coatings (typically greater than 0.18 mm in depth) for the outer surface of the cladding.
These conventional solutions, however, have several drawbacks. The first category of solution involves improving the silica itself by integrating reagent particles (e.g., titanium) into the outer cladding itself. However, these hard macroscopic films require considerably force and tension for cleaving.
The second category of solutions include putting micron-sized metal oxide particles (e.g., titania, silica, or alumina) into the protective polymer to retard corrosion. The limitations associated with this method could include micro bending losses, polymer degradation, and poor adhesion of coating to silica. A micro bend is a distortion or abrupt bend in the core-cladding caused by localized stresses (i.e., a site where a metal oxide particle imparts the core-cladding assembly). When a bend is abrupt, the light angle of incidence at the internal surface becomes large and is no longer substantially reflected, thus causing the loss of transmission power.
With the third category of conventional solutions, even with good adhesion of optical fiber protective layers to silica, environmental fluids will be absorbed and transmitted under high temperature, pressure, or humidity to the core-cladding assembly of the fiber. Thus the slow, but nevertheless persistent, penetration of these environmental substances through the outer protective layers will, over time, result in the corrosion of the core-cladding assembly destroying not only its tensile strength, but its optical transmission properties.
The fourth class conventional solution generally involves one of two processes: the pyrolysis of titanium tetrachloride, hydrogen, and oxygen which is blasted at the fiber as it is drawn, creating macroscopic film on the surface of the cladding or depositing a film of titanium silicate glass (5 microns in depth). In addition to being still susceptible to environmental fluids with aging, the pyrolysis process is generally not pragmatic since it requires high energy, temperature, and pressure with these very reactive gasses.